For many years conventional magnetic storage devices have been used to store data and information. Magnetic storage devices generally include units of magnetic material that can be polarized to distinct magnetic states, where the direction of the magnetization points in different directions and those different directions can be referred to as a positive state and a negative state. In this application we will refer to those different directions of magnetization as corresponding to different polarities. Each bit can store information (generally binary information in the form of either a 1 or a 0) according to the magnetic polarization state of the bit. Accordingly, magnetic storage devices generally include a “read” element that passes over the magnetic material and perceives the magnetic polarization state of each bit and a “write” element that passes over the magnetic material and changes the magnetic polarization state of each bit, thereby recording individual units of information. Therefore, the amount of information that can be stored on a magnetic storage device is directly proportional to the number of magnetic bits on the magnetic storage device.
There are various types of magnetic storage devices and each type involves different fabrication techniques. For example, conventional granular magnetic recording devices are disks that have multiple grains in each magnetic bit. In granular magnetic devices, all of the domains are co-planar and the surface of the disk is relatively smooth and continuous. In order to increase the amount of information that can be stored on a granular magnetic disk, the number of grains per magnetic bit can be decreased while keeping the grain size approximately the same. However, with fewer grains in each bit, there is decreased signal to noise ratio (less signal, more noise). In order to maintain a better signal to noise ratio, methods have been developed that decrease both the size of the magnetic bit and the size of the individual grains making up each magnetic bit, thus keeping the same number of grains in each magnetic bit. However, when the grains become too small, thermal fluctuations can cause the grains to spontaneously reverse polarity, thus resulting in unstable storage and a loss of information.
Bit-patterned media (BPM) is another example of magnetic storage media. In bit-patterned media, each bit is a single domain magnetic island or region, rather than a collection of contiguous magnetic grains. The BPM bits can be topographically patterned using lithographic and etching techniques to form magnetically isolated bit islands surrounded by trenches. In some instances, the trenches are formed by etching away a magnetic material and in other instances the physical patterns are etched into a non-magnetic substrate followed by a deposition of magnetic material over the patterned substrate. Because of the physical separation between the elevated bit islands and the trenches, the width of each distinct bit island can be decreased in order to increase the areal bit density of the device while still maintaining a high signal to noise ratio and high thermal stability.
However, etching bit-patterned media with bit densities exceeding one trillion bits per square inch (Tbit/in2) pushes the resolution of conventional etching techniques to their limits. Such etching techniques generally include covering a portion of the material to be etched with a mask or a protective material that shields portions of the underlying material from the etching process—i.e. the mask protects the islands while leaving exposed the portions of the material that will be etched to form the trenches. Such etching processes degrade the peripheral walls of the islands. This damage to the walls of the islands may decrease the magnetic anisotropy of the magnetic material at the islands' periphery, thus increasing the likelihood of spontaneous magnetic switching initiated at the peripheral edges of the islands.
Thus, conventional bit-patterned media is susceptible to spontaneous magnetic reversal caused by adjacent track exposure. In other words, as the read/write head passes over a particular track of nano-sized islands on a bit-patterned medium, occasionally the magnetic field from the read/write head will interact with the peripheral edges of magnetic islands in adjacent tracks. This inadvertent interaction with adjacent tracks may initiate magnetic switching in adjacent islands and may therefore cause a loss or corruption of information stored on the bit-patterned magnetic recording device.